Diffraction Grating Based Interferometric Systems And Methods

ABSTRACT

Diffraction grating based fiber optic interferometric systems for use in optical coherence tomography, wherein sample and reference light beams are formed by a first beam splitter and the sample light beam received from a sample and a reference light beam are combined on a second beam splitter. In one embodiment, the first beam splitter is an approximately 50/50 beam splitter, and the second beam splitter is a non 50/50 beam splitter. More than half of the energy of the sample light beam is directed into the combined beam and less than half of the energy of the reference light beam are directed into the combined beam by the second beam splitter. In another embodiment, the first beam splitter is a non 50/50 beam splitter and the second beam splitter is an approximately 50/50 beam splitter. An optical circulator is provided to enable the sample light beam to bypass the first beam splitter after interaction with a sample. Two combined beams are formed by the second beam splitter for detection by two respective detectors. More than half of the energy of the light source provided to the first beam splitter is directed into the sample light beam and less than half of the energy is directed into the reference light beam. The energy distribution between the sample and reference light beams can be controlled by selection of the characteristics of the beam splitters.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/166,522, filed Jun. 24, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/017,534, filed Oct. 18, 2001, now U.S. Pat. No.7,006,231, which is related to U.S. patent application Ser. No.10/020,040, filed, Oct. 18, 2001, now U.S. Pat. No. 6,847,454, all ofwhich are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to diffraction grating basedinterferometers and, more particularly, to diffraction grating basedinterferometric systems for use in optical coherence tomography.

BACKGROUND OF THE INVENTION

Optical Coherence Tomography (“OCT”) is a type of opticalcoherence-domain reflectometry that uses low coherence interferometry toperform high resolution ranging and cross-sectional imaging. In OCTsystems, a light beam from a low coherence light source is split into areference light beam and a sample light beam. A diffraction grating maybe used to provide an optical path difference in one or both lightbeams. The sample light beam is directed onto a sample and the lightscattered from the sample is combined with the reference light beam. Thecombination of the sample and reference light beams results in aninterference pattern corresponding to the variation in the samplereflection with the depth of the sample, along the sample beam. Thesample beam typically suffers a high loss of energy due to itsinteraction with the sample. The reference beam serves as a localoscillator to amplify the interference pattern to a detectable level andtherefore must have a much higher energy level than the sample lightbeam. The interference pattern is detected by a photo detector, whoseoutput is processed to generate a cross-sectional image of the sample.High resolution (less than 10 micrometer) imaging of the cross-sectionsof the sample by OCT is useful in biological and medical examinationsand procedures, as well as in materials and manufacturing applications.

OCT based systems may be implemented with fiber optics and an opticalfiber carrying the sample light beam may be incorporated into a catheteror an endoscope for insertion into internal body cavities and organs,such as blood vessels, the gastrointestinal tract, the gynecologicaltract and the bladder, to generate images of internal cross-sections ofthe cavities or organs. The sample beam is typically emitted from thedistal end of the instrument, where a prism or a mirror, for example,directs the sample light beam towards a wall of the cavity. The opticalfiber and the prism or mirror may be rotated by a motor to facilitateexamination of the circumference of the cavity.

An example of a fiber optic OCT system is shown in U.S. Pat. No.5,943,133 (“the '133 patent”), where sample and reference light beamsare carried in respective optical fibers to a diffraction grating, whichintroduces an optical path difference across the light beams and alsocombines the sample and reference light beams. FIG. 1 is a schematicdiagram of a system 10 disclosed in the '133 patent. The system includesa light source 12 optically coupled to a 50/50 beam splitter 14 throughan optical fiber 16. The beam splitter 14 splits the incident light beamequally into a sample light beam and a reference light beam. The samplelight beam is carried by an optical fiber 18 to a focusing lens 20,which focuses the sample light beam onto a sample 22. The optical fiber18 may be contained within a catheter (not shown) for insertion into abody cavity, such as a blood vessel, for examination of the tissue ofthe wall of the cavity. Light received from the tissue is focused by thelens 20 and coupled back into the optical fiber. The received lighttravels back to the beam splitter 14, where it is split again. A portionof the received light is directed into another optical fiber 24, whichconveys the light to a first collimator 26. The reference light beamtravels through an optical fiber 28 to a second collimator 30. The firstand second collimators 26, 30 direct the sample and reference lightbeams onto the same region of a diffraction grating 32. The diffracted,combined light beam is conjugated on the detector plane of amulti-channel linear diode array detector 34 by a conjugating 36 lens. Aneutral density filter (not shown) is provided to decrease the energy inthe reference beam to prevent saturation of the detector.

The sample light beam suffers a significant loss of energy due to itsinteraction with the sample. The second pass through the 50/50 beamsplitter further reduces the already attenuated light beam. In addition,the interaction of the light beams with the diffraction grating causes afurther loss in both the sample light beam and the reference light beamof about 50% of the incident light in the first order. The diffractiongrating also introduces noise. As a result, the system of the '133patent has a low signal-to-noise ratio.

Another interferometric system using a diffraction grating is describedin “Nonmechanical grating-generated scanning coherence microscopy”,Optics Letters, Vol. 23, No. 23, Dec. 1, 1998. FIG. 2 is a schematicdiagram of the disclosed system 50. A light source 52 provides light toa 50/50 beam splitter 54 that splits the energy in the light beamequally into a sample light beam 55 and a reference light beam 56. Thesample light beam 55 is directed to a focusing lens 58 that focuses thesample light beam onto a sample 60. The light received by the focusinglens 58 from the sample 60 is returned to the beam splitter 54. Thereference light beam 56 is directed to a diffraction grating 62 in aLittrow configuration, which introduces an optical path differenceacross the reference light beam. The diffracted reference light beam isalso returned to the beam splitter 54. The sample and reference lightbeams are then combined in the beam splitter 54 and directed to acharge-coupled device (CCD) array 64 for detection and processing by acomputer 66. The reference light beam needs to be suppressed here, aswell.

Here, only the reference light beam is diffracted, making the system 50more efficient than the system 10 of the '133 patent, shown in FIG. 1.However, the sample and reference arms in the system 50 of FIG. 2 cannotboth be implemented with fiber optics. The diffraction gratingintroduces an optical path difference across the width of the beam. Thedetector is a multi-element detector at least as wide as the light beamand each element of the detector receives a portion of the beamcorresponding to its position on the diffraction grating. If thereference light beam is conveyed by an optical fiber from thediffraction grating to the detector, the spatial order is lost. If thesample arm is implemented in fiber optics but the reference arm is not,the length of the open space reference arm would be inconveniently long.

In OCT systems, either the reference light beam or the sample light beammay be modulated to provide a relatively low frequency beating used as acarrier frequency. The mechanical motion may be used to scan the opticalpath, which essentially represents the sample depth. This motion alsocreates a Doppler frequency shift. A moving or oscillating mirror and afiber stretcher, such as a piezoelectric stretcher, are commonly usedfor mechanically modulating the light. One or a pair of acousto-opticmodulators may also be used to modulate the light beam, as described inU.S. Pat. No. 5,321,501, for example. The amplitude of the frequency ofmodulation is modulated by the intensity of the reflected and scatteredlight in the sample beam. The signal is then processed using a narrowband amplifier tuned to the frequency, to extract the intensityvariation to produce an image.

In diffraction grating based interferometry using a multi-element photodetector, scanning the depth is typically not necessary because thedepth is instantly projected onto the multi-element photo detector.Depending on the signal processing method, however, there may be a needfor low frequency modulation. For example, if the detector is a photodiode array and heterodyne signal processing is used, low frequencymodulation is required. Providing a separate modulating unit in theinterferometer takes up additional space and adds to the complexity ofthe system. If the detector is a charge coupled device (CCD), modulationis not needed.

In prior art diffraction grating based OCT systems, the sample lightbeam typically passes through the beam splitter that creates the sampleand the reference light beams, twice. It is therefore most efficient touse a 50/50 beam splitter that directs half of the energy from the lightsource into the reference beam and half of the energy into the samplebeam. However, much of the energy in the reference light beam needs tobe suppressed to prevent saturation of the detector. Such energy is lostin the system. The sample light beam, which suffers high loss due to itsinteraction with the sample as well as the second pass through the beamsplitter, only receives half of the energy of the light source. Thesample light beam also suffers loss and noise if it is diffracted by thediffraction grating. A more efficient diffraction grating basedinterferometer for use in OCT systems would be advantageous. A moreefficient diffraction based interferometer, where the sample andreference light beams are carried by optical fibers, would also beadvantageous.

SUMMARY OF THE INVENTION

In one embodiment of the invention, an interferometer is disclosedcomprising a low coherence light source and a first beam splitter incommunication with the light source to split light from the light sourceinto a first sample light beam to be directed onto a sample and areference light beam. Light received from the sample forms a secondsample light beam. A diffraction grating is positioned to diffract atleast one of the reference light beam or the second sample light beam.The diffraction grating introduces an optical path difference across thediffracted light beam. A second beam splitter is positioned to receivethe second sample light beam and the reference light beam, after atleast one of those beams has been diffracted. The second sample lightbeam and the reference light beam are combined in the second beamsplitter to form a combined light beam. A detector is positioned toreceive the combined light beam from the second beam splitter.Preferably, the detector is a multi-element detector. A signalprocessor, such as a computer, processes the output from the detectorinto an image for display.

Preferably, the reference light beam is diffracted and the sample lightbeam is directed onto the second beam splitter without being diffracted.By only diffracting the reference light beam and combining the samplelight beam with the diffracted reference light beam on a second beamsplitter, the sample light beam does not suffer from loss and noise dueto interacting with the diffraction grating.

In one variation of this embodiment, the second beam splitter is a non50/50 beam splitter. The first beam splitter may be an approximately50/50 beam splitter and the characteristics of the second beam splittermay be adjusted so that a sufficient amount of energy is provided by thereference beam to amplify the sample beam for analysis, withoutsaturating the detector. For example, the second beam splitter maydirect more than half of the light energy of the second sample lightbeam into the combined beam and less than half of the light energy ofthe reference light beam into the combined beam. Preferably, the secondbeam splitter directs substantially more than half of the light energyof the second sample light beam and substantially less than half of thelight energy of the reference light beam into the combined beam. Morepreferably, the second beam splitter directs at least about 90% of thelight energy of the second sample light beam into the combined lightbeam and directs about 10% or less of the light energy of the referencelight beam into the combined light beam.

In another variation of this embodiment, the first beam splitter is anon 50/50 beam splitter. The first beam splitter directs more than halfof the light energy received from the light source into the sample lightbeam and less than half of the light energy received from the lightsource into the reference light beam. An optical circulator may beprovided to direct the sample light beam to the sample and to direct thesecond sample light beam from the sample to the second beam splitter.Use of an optical circulator enables the light received from the sampleunder examination to bypass the first beam splitter. The first beamsplitter need not, therefore, be a 50/50 beam splitter, and itscharacteristics may be adjusted to optimize the energy distributionbetween the sample and reference light beams. For example, substantiallymore than half of the light energy received from the light source ispreferably directed into the sample light beam and substantially lessthan half of the light energy received from the light source is directedinto the reference light beam. More preferably, at least about 90% ofthe light energy received from the light source is directed into thesample light beam and about 10% or less of the light energy receivedfrom the light source into the reference light beam.

The second beam splitter may be an approximately 50/50 beam splitter andthe second sample light beam and the reference beam may be combined inthe second beam splitter to form first and second combined light beams.In that case, the first light beam may be detected by the first detectorand a second detector may be provided to detect the second light beam.

In another embodiment of the invention, first and second low coherencelight sources are provided in an interferometer, each emitting light ata different wavelength. A first beam splitter receives the light fromthe light sources and forms sample and reference light beams. The samplelight beam is directed onto a sample and light received from the sampleforms a second sample light beam. At least one of the reference lightbeam or the second sample light beam is diffracted. A second beamsplitter forms two combined light beams from the reference light beamand the second sample light beam and two detectors are provided, one todetect each beam.

In another embodiment of the invention, an interferometer comprises abeam splitter that forms two combined light beams for detection by twodetectors. Polarization filters having different polarizations areprovided between the beam splitter and each detector. Birefrigencemeasurements may thereby be made.

In another embodiment of the invention, a fiber optic interferometer isdisclosed wherein the sample and reference light beams are combined on abeam splitter. A first fiber optic beam splitter splits the lightreceived from a light source along an optical fiber into a sample lightbeam and a reference light beam. The sample light beam is conveyed fromthe beam splitter to a sample by another optical fiber. The lightreceived from the sample is coupled back into the optical fiber, andreturned to the fiber optic beam splitter. The light received from thesample is conveyed from the fiber optic beam splitter to a second beamsplitter by another optical fiber. Meanwhile, the reference light beamis conveyed from the fiber optic beam splitter to a diffraction gratingby another optical fiber. The diffraction grating introduces an opticalpath difference across the reference light beam. The diffraction gratingdirects a diffracted reference light beam to the second beam splitter,where it combines with the second sample light beam. The combined lightbeam is directed toward a detector for detection. Preferably, thedetector is a multi-element photo detector. A signal processor processesthe output of the detector into an image for display on a monitor, forexample.

By providing two beam splitters, one to form the sample and referencelight beams and the other to combine the sample and reference lightbeams, the sample light beam may be carried by optical fibers to thesample to be analyzed and to the second beam splitter while thereference light beam may be carried by an optical fiber to thediffraction grating. A fiber optic OCT system may thereby beimplemented.

Preferably, the first beam splitter is an approximately 50/50 beamsplitter and the second beam splitter is a non 50/50 beam splitter. Thesecond, non 50/50 beam splitter directs more than half, and preferablysubstantially more than half, of the energy of the sample light beam andless than half, and preferably substantially less than half, of theenergy of the diffracted reference light beam into the combined beam.More preferably, at least about 90% of the light energy is directed intothe sample light beam and about 10% or less of the light energy isdirected into the reference light beam.

In another embodiment of the invention implemented with fiber optics,the first beam splitter, that splits the light from the light sourceinto a sample and reference beam, is a non 50/50 beam splitter. Morethan half, and preferably substantially more than half, of the lightenergy of the light received from the source is directed into the samplelight beam and less than half, and preferably substantially less thanhalf, of the light energy is directed into the reference light beam.More preferably, at least about 90% of the light energy is directed intothe sample light beam and about 10% or less of the light energy isdirected into the reference light beam. The sample light beam isprovided from the first beam splitter to an optical circulator by anoptical fiber. Another optical fiber conveys the sample light beam tothe sample to be analyzed. Light received from the sample is conveyedback to the optical circulator by the same optical fiber. The receivedlight is then conveyed from the optical circulator to a second beamsplitter. Meanwhile, the reference light beam is conveyed by an opticalfiber to a diffraction grating. The diffraction grating reflects thediffracted reference light beam onto the second beam splitter, forcombination with the sample light beam.

The second beam splitter may be a 50/50 beam splitter. Two combinedlight beams with the same proportion of energy from the sample andreference light beams are thereby formed, which may be detected by twodetectors. Preferably, the detector detectors are each multi-elementphoto detectors. A signal processor processes the output of the detectoror detectors into an image for display.

Alternatively, the second beam splitter may also be a non 50/50 beamsplitter that directs more than half of the energy of the sample lightbeam and less than half of the energy of the diffracted reference lightbeam into a combined beam directed toward a single detector, which isalso preferably a multi-element photo detector. The characteristics ofthe second beam splitter may also be adjusted so that a sufficientamount of energy is provided by the reference beam to amplify the samplebeam for analysis, without saturating the detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art OCT system;

FIG. 2 is a schematic diagram of another prior art OCT system;

FIG. 3 a is a schematic diagram of a diffraction grating based fiberoptic interferometric system in accordance with one embodiment of theinvention;

FIG. 3 b is a schematic diagram of an interferometric system with asimilar arrangement as the system of FIG. 3 a, where both beam splittersare 50/50 beam splitters;

FIG. 4 is a schematic diagram of an interferometric system with asimilar arrangement as the system of FIG. 3 a, where the diffractiongrating is a transparent diffraction grating;

FIG. 5 is a schematic diagram of another embodiment of the invention,including an optical circulator and a first, non 50/50 beam splitter;

FIG. 6 is a schematic diagram of the system of FIG. 5, includingpolarization filters for use in detecting polarization relatedinformation;

FIG. 7 is a schematic diagram of the system of FIG. 5, includingmultiple light sources;

FIG. 8 is a schematic diagram of the system of FIG. 5, including anoptical circulator and two non 50/50 beam splitters;

FIG. 9 is an enlarged view of the reference light beam being diffractedby the diffraction grating, indicating the optical path differenceacross the reference beam;

FIG. 10 is a schematic diagram of an interferometric system inaccordance with another embodiment of the invention, wherein anacousto-optic modulator (“AOM”) acts as both a transparent diffractiongrating to introduce an optical path difference and as a modulator;

FIG. 11 is a schematic diagram of an interferometer connected to anultrasound console;

FIG. 12 is a schematic diagram of an AOM based interferometric system,as in the embodiment of FIG. 10, coupled to an ultrasound console; and

FIG. 13 shows an interferometric system in accordance with theembodiment of FIG. 3 a contained within a housing for use with ainterferometric catheter and an ultrasound console.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 a is a schematic diagram of one embodiment of a diffractiongrating based fiber optic interferometric system 100. The system 100comprises a light source 102 optically coupled to a fiber optic beamsplitter 104 by an optical fiber 106. The fiber optic beam splitter 104is preferably approximately a 50/50 beam splitter. More preferably, thebeam splitter 104 is a 50/50 beam splitter. An optical fiber 108 isoptically coupled to the fiber optic beam splitter and to a focusinglens 110.

An optical fiber 111 is also optically coupled to the fiber optic beamsplitter 104 such that light entering the beam splitter from the opticalfiber 108 is coupled into the optical fiber 110. The optical fiber 111is also optically coupled to a first collimator 112. Another opticalfiber 114 is optically coupled to the first beam splitter 104 and to asecond collimator 116.

The optical fibers 108 and 110 comprise first and second parts of asample arm, respectively, of the interferometric system 100. The opticalfiber 114 comprises a reference arm of the system 100. Light from thelight source 102 passes through the fiber optic beam splitter 104 and issplit into a sample light beam and a reference light beam, each havinghalf of the energy of the initial light beam provided from the lightsource 102 to the fiber optic beam splitter 104. The sample light beamis directed into the optical fiber 108 of the first part of the samplearm and the reference light beam is directed into the optical fiber 114of the reference arm. The sample light beam is focused by the focusinglens 110 onto a sample of interest 119, which may be tissue within abody cavity, for example. Light scattered by the sample is focused bythe focusing lens 110 to form a second sample light beam and is coupledback into the optical fiber 108 of the sample arm. That light passesback through the first beam splitter 104, where the light beam is splitagain. A light beam having half of the energy of the received light beamis coupled into the optical fiber 110 of the second part of the samplearm.

The second collimator 116 collimates the reference light beam anddirects the reference light beam onto a diffraction grating 118 at anangle α. The diffraction grating 118 introduces an optical pathdifference to the reference light beam and reflects the diffractedreference light beam onto a second, open space beam splitter 120. Thefirst collimator 112 also collimates the second sample light beam anddirects it onto the second beam splitter 120.

The second beam splitter 120 combines the second sample light beam andthe reference light beam and directs a portion of the combined lightbeam onto a photo detector 122, through a conjugating lens 124. Thephoto detector 112 is preferably a multi-element photo detector, such asa photo diode array. An array of avalanche mode photo diodes may beused, for example. A charge coupled-device (“CCD”) may be used, as well.The conjugating lens 124 projects the image of the combined beam on theplane of the second beam splitter onto the detector plane. The detector122 is connected to a signal processor, such as a computer 126, whichprocesses the data received from the detectors to create an image on adisplay 128, such as a monitor. The output of the detector 122 may beconverted into a digital signal prior to being input to the computer126. The image may be printed as well.

In accordance with this first embodiment, the open space beam splitter120 directs less than half of the light energy in the reference beam andmore than half of the energy in the second sample light beam into thecombined beam directed toward the detector 122. Preferably,substantially more than half of the energy in the second sample lightbeam, such as 75% or more, is directed into the combined beam andsubstantially less than half of the energy in the reference light beam,such as 25% or less, is directed into the combined light beam. Morepreferably, at least about 90% or more of the energy of the sample lightbeam and about 10% or less of the energy of the reference light beam aredirected into the combined beam. For example, the second beam splittermay be a 10/90, 5/95, 2/98 or 1/99 beam splitter. In the embodiment ofFIG. 3 a, the reference light beam is transmitted through the secondbeam splitter 120 while the sample light beam is reflected by the secondbeam splitter 120. Alternatively, the sample light beam may betransmitted through the second beam splitter 120 and the reference lightbeam may be reflected by the second beam splitter.

As is known in the art, in order for there to be constructiveinterference between the sample and reference light beams in this andthe other embodiments of the invention, the optical path lengths of thesample light beam (the initial sample light beam and the second lightbeam) and the reference light beam from the first beam splitter 104 tothe second beam splitter 120 need to be equal to within the coherencelength of the light source 102. The refractive index of the opticalfibers and the open space traversed by the light beams, as well as therefractive index of the sample material, need to be considered indetermining appropriate path lengths.

The interference pattern resulting from the combination of the sampleand reference light beams contains both depth information and brightnessinformation. The brightness information is provided by the lightintensities of the interference pattern. Since the portion of the secondsample light beam that is received in the sample arm from a certaindepth from the sample interferes with a portion of the diffractedreference beam at a spatial position corresponding to the optical pathdifference for this position, the depth information is provided by thespatial position within the interference pattern. The photo detectors ofthe array 122 are arranged so that each photo detector element detectsthe light intensity of the interference pattern at a certain spatialposition within the interference pattern, as is known in the art. Thus,the output of each photo detector element provides image brightnessinformation for a certain image depth. The array 122 outputs theinformation along parallel channels (not shown), where each channelcorresponds to the output of one of the photo detector elements. Theoutputs of the parallel channels of the photo array 122 are provided tothe computer 126 for processing in accordance with known processingtechniques to produce an image of the sample depth reflection along thesample light beam for display. Preferably, the multi-element detector122 is a photo diode array and a heterodyne detection technique is used.

As discussed above, if the detector is a photo diode array and aheterodyne detection method is used, low frequency modulation isrequired. A modulator 117, such as a fiber stretcher or an acousto-opticmodulator, is therefore provided along the optical fiber 114. Themodulator 117 may be provided along the optical fibers 108 or 111 tomodulate the sample light beam, as well.

Use of two beam splitters enables the reference light beam to beconveyed to the diffraction grating 118 by an optical fiber 114. Sincethe second sample light beam is not combined with the reference lightbeam on the diffraction grating in this embodiment, additional loss andnoise is not introduced to the second sample light beam. Since theoptical path from the diffraction grating 118 to the detector 122 isopen space, spatial information in the reference and sample light beamsis preserved.

The optical fiber 108 of the first part of the sample arm is preferablyincorporated in a catheter adapted to be positioned in a body cavity ororgan by standard catheter intervention procedures. For example, thecatheter may be inserted into a blood vessel or the heart by guiding theflexible catheter through various blood vessels along a circuitous path,starting, for example, by percutaneous introduction through anintroducer sheath disposed in a perforation of the femoral artery.Alternatively, the catheter can be introduced directly into a bodycavity or body tissue, such as an organ. The optical fiber may becoupled to a motor for causing rotation of the fiber within thecatheter. Catheters and endoscopes for use in the optical imaging ofblood vessels and other internal body cavities are known in the art andare described in U.S. Pat. No. 6,134,003, U.S. Pat. No. 5,321,501 andInternational Publication No. WO 98/38907 published on Sep. 11, 1998,for example, which are incorporated by reference herein. As disclosed inthose references, a mirror or prism may be provided to reflect thesample light beam onto biological tissue parallel to the optical fiberand to reflect light received from the tissue into the optical fiber. Byrotating the optical fiber, tissue along the circumference of the cavitymay be examined.

While it is preferred that the second beam splitter 120 in FIG. 3 a be anon 50/50 beam splitter, it is not required. FIG. 3 b is a schematicdiagram of an interferometric system 100′ that is similar to the systemof FIG. 3 a, except that the second beam splitter 120′ is a 50/50 beamsplitter. A neutral density filter or other such attenuator may beprovided as needed to suppress the reference light beam to preventsaturation of the detector 124. Components common to the embodiment ofFIG. 3 a are commonly numbered.

FIG. 4 is a schematic diagram of an interferometric system 150 with asimilar arrangement to the system 100 of FIG. 3 a, except that thediffraction grating is a transparent diffraction grating 152. Componentscommon to the configuration of FIG. 3 a are commonly numbered in FIG. 4.The second collimator 116 is arranged to direct the reference light beamon a rear side of the diffraction grating 152 at an angle α. Thediffracted reference beam is directed onto the open space beam splitter120, for combination with the second sample light beam, as discussedabove. The combined light beam is directed through the conjugating lens124 and onto the multi-element detector 122, also as described above.The ability to use either a reflective diffraction grating 118 or atransparent diffraction grating 152 in the interferometric systems ofthe invention, adds flexibility to the design of the interferometer inpractical applications. Any of the embodiments described herein can useeither a reflective or a transparent diffraction grating.

FIG. 5 is a schematic diagram of another embodiment of aninterferometric system 200, wherein more than half of the light energyis directed into the sample light beam and less than half of the lightenergy is directed into the reference light beam by use of a non 50/50fiber optic beam splitter. Preferably, substantially more than half ofthe light energy incident on the beam splitter, such as 75% of theenergy, is directed into the sample light beam and substantially lessthan half of the incident light energy, such as 25%, is directed intothe reference light beam. More preferably, at least about 90% of theincident light energy is directed into the sample light beam and about10% or less is directed into the reference light beam.

In this embodiment, the sample light beam is directed to and from thesample under examination through an optical circulator instead of a beamsplitter, as in the embodiment of FIG. 3 a and in the prior art of FIGS.1 and 2. Therefore, the first beam splitter need not be approximately a50/50 beam splitter.

Components common to the embodiment of FIG. 3 a are commonly numbered inFIG. 5. A light source 102 provides light to a 90/10 beam splitter 202through an optical fiber 106. The 90/10 fiber optic beam splitter 202provides 90% of the energy of the light incident to the beam splitter202 into the sample light beam and 10% of the energy of the light intothe reference light beam.

An optical circulator 204 is provided with three ports, Port 1, Port 2and Port 3. Light entering the optical circulator 204 through Port 1 isdirected out of the circulator through Port 2. Light entering theoptical circulator 204 through Port 2 is directed out of the circulatorthrough Port 3. An optical fiber 206 is optically coupled to the firstbeam splitter 202 to Port 1 of the optical circulator 204 to convey thesample light beam to the circulator.

An optical fiber 208 is optically coupled to Port 2 of the opticalcirculator 204 and to a focusing lens 110. An optical fiber 210 isoptically coupled to Port 3 of the optical circulator 204 and to a firstcollimator 112. The sample light beam is conveyed from the first beamsplitter 202 to Port 1 of the optical circulator 204 through the opticalfiber 206. The sample light beam is directed to Port 2 of the opticalcirculator, where it exits the circulator and is conveyed to thefocusing lens 110 by the optical fiber 208. The focusing lens focusesthe sample light beam onto the sample 119. Light received from thesample is focused and coupled into the optical fiber 108, forming asecond sample light beam to be returned to Port 2 of the opticalcirculator. The second sample light beam is directed from Port 2 to Port3 of the optical circulator, where it is conveyed by the optical fiber204 to the first collimator 112.

An optical fiber 220 is also optically coupled to the beam splitter 202and to a second collimator 116, as in the embodiment of FIG. 3 a. Areference light beam having 10% of the energy of the light conveyed tothe 90/10 beam splitter 202 from the light source 102 is directed intothe optical fiber 220. The second collimator 116 directs the referencelight beam onto a reflective diffraction grating 118. The diffractiongrating 118 introduces an optical path difference to the reference lightbeam and reflects the diffracted reference light beam onto the openspace beam splitter 120. A transparent diffraction grating 152 could beused instead of the reflective diffraction grating 118, as discussedabove. The first collimator 112 also directs the second sample lightbeam onto the open space beam splitter 120 for combination with thereference light beam.

In this embodiment, the second beam splitter 120 is approximately a50/50 beam splitter 222. Preferably, the second beam splitter 120 is a50/50 beam splitter. Two combined sample/reference beams, each havinghalf of the energy of the second sample light beam and half of theenergy of the reference light beam, are formed. Two photo detectors 224,226, which are preferably multi-element photo detectors, are provided,one along the path of each combined light beam. Because two detectorsare provided, the 50/50 beam splitter 222 does not cause a loss ofenergy and information in the second sample light beam. Respectiveconjugating lenses 228, 230 are provided between each detector 224, 226and the second beam splitter 222. The outputs of individual detectors incorresponding spatial positions in each array are combined by analogcircuitry 227. The output of the analog circuitry 227, which may beparallel or serial, is provided to a signal processor, such as thecomputer 126, for processing into an image in a manner known in the art.As noted above, the analog circuitry 227 may convert the signals outputfrom the detectors 224, 226 into digital signals, as well. Two detectorsmay be readily provided in the embodiment of FIG. 3 b, as well, in thesame manner.

Preferably, about 90% or more of the light energy is directed into thesample light beam and about 10% or less of the light energy is directedinto the reference light beam by the first beam splitter 202. The amountof energy provided to the sample and reference beams may be controlledby selection of the characteristics of the fiber optic beam splitter 202so that only the necessary amount of light energy is provided to thereference light beam to sufficiently amplify the sample light beam forimaging without saturating the multi-element photo detectors 224, 226.The remainder of the energy is directed to the sample light beam. A2/98, a 95/5 or a 1/99 or other such beam splitter may also be used, forexample.

Directing the second sample light beam received from the sample 119through the optical 204 circulator 204 instead of back through the firstbeam splitter 202 avoids a significant source of loss in the secondsample beam. The loss in the optical circulator is between about 0.5decibels (“db”) to about 1.1 db each way. The two way loss in theoptical circulator is therefore about 1.0 db to about 2.2 db (about37%). The loss in a 50/50 beam splitter 222, by contrast, is 50% eachway or 75% if the sample beam travels through the 50/50 beam splittertwice.

The detectors 224, 226 may be tuned to detect light at the samewavelength band or at different wavelength bands. The ability to detectmore than one wavelength band is useful for spectroscopy and forreducing aliasing in the image.

The two combined sample/reference light beams in the embodiment of FIG.5 may contain polarization related information. Birefringencemeasurements may be made by providing a polarization filter along eachlight beam, where each filter allows passage of light having a differentpolarization. In FIG. 6, polarization filters 240, 242 are shown betweeneach of the conjugating lenses 228, 230 and the detectors 224, 226,respectively. The outputs of each detector 222, 224 may be provided tothe computer separately, for processing. Two images may be displayed.Differential measurements may be made by comparing the signals at eachdetector as a function of spatial position and relative intensity, as isalso known in the art. Variations in intensity versus position are anindication of polarity sensitive areas of target tissue. The opticalfiber used in this embodiment is preferably a polarization maintaining(high birefringence) optical fiber, as is known in the art. Apolarization filter 243, shown in phantom, may also be provided betweenthe light source 102 and the fiber optic beam splitter 202 instead ofthe polarization filters 240, 242, to polarize the light beam emitted bythe light source to a desired polarization. Instead of the polarizationfilter 243, the second beam splitter 222 may be a polarization beamsplitter. A single detector, as in the embodiments of FIGS. 3 and 4, mayalso be used to detect a light beam of a particular polarization.

Polarization filters may be provided in other interferometric systemswhere two combined beams are formed, as well. For example, a 50/50 beamsplitter may also be provided between the diffraction grating 32 and thedetector 34 in the system of the '133 patent shown in FIG. 1, to formtwo combined beams. A second detector, two polarization filters and twoconjugating lenses may then be provided, as in FIG. 6, to conductbirefrigence measures.

In another variation in the embodiment of FIG. 5, a second light source103 may be provided, as shown in FIG. 7. Additional light sources mayalso be provided. Each light source may emit light at a differentwavelength. For example, the first light source can emit light at 800nanometers and the second light source can emit light at 1200nanometers. The light from the second light source 103 may be coupledinto the optical fiber 106 by a wavelength division multiplexor, forexample. One of the detectors 224, 226 may be tuned to detect light at awavelength corresponding to the first light source 102 and the otherdetector may be tuned to detect light at a wavelength band correspondingto the second light source 103. If more than two light sources areprovided, the individual photo detectors in each array can be tuned todetect light at different wavelength bands. Bandpass filtering, detectorresponse and the fiber characteristics of each “detection channel” maybe selected to optimize the use of specific wavelengths. The outputs ofeach detector 222, 224 may be provided to the computer separately forprocessing. Two or more images may be displayed. The interferencepatterns at each wavelength band may be compared as a function ofspatial position and intensity at each wavelength band. The differencein intensity at the same position in the interference patterns mayindicate wavelength dependent attenuation or absorption of the sample.

Fluorescence of tissue is known to be dependant upon tissue type andtissue constituents. One of the light sources in FIG. 7 may be in theblue or ultraviolet range, for example, to induce fluorescence in thetissue. One of the detectors 224, 226 may be tuned to the ultraviolet,blue or other wavelength band at which the target tissue is expected tofluoresce to detect the intensity of the emitted fluorescent light.

In another embodiment using an optical circulator 204, neither the firstfiber optic beam splitter nor the second open space beam splitter is a50/50 beam splitter. In the system 280 of FIG. 8, the first beamsplitter 282 is a 95/5 beam splitter, for example, that directs 95% ofthe light energy provided to the beam splitter into the sample lightbeam and 10% into the reference light beam. The second open space beamsplitter 284 is a 10/90 beam splitter, for example, directing 90% of thelight energy in the second sample light beam and 10% or less of thelight energy in the reference light beam toward a single detector 286 inthe combined beam. Varying the characteristics of both beam splitters282, 284 provides additional flexibility in optimizing the energydistribution between the sample and reference light beams. Components ofthe system 280 common to the embodiments of FIGS. 5 and 3 a are commonlynumbered.

To determine the theoretical percentage of the light source energyreaching the detector from the sample arm in the various embodiments ofthe invention and in the prior art, the sample under examination may bereplaced by a mirror. The Table below shows the percentage of the lightsource energy in the sample and reference arms at the sample, at thediffraction grating and at the detector in the prior art interferometerof FIG. 1 and in the example interferometers of FIGS. 3 a, 4 and 5, ifthe sample light beam is reflected by a mirror (suffers no loss due tointeraction with the sample). Percentage of light source energy incidenton: Sample Diffraction Grating Detector/Detectors Sample SampleReference Sample Reference Figure Arm Arm Arm Arm Arm 50 25 50 12.5 2550 NA 50 22.5 2.5 50 NA 50 22.5 2.5 90 NA 10 56.7 5

In the prior art of FIG. 1, the light energy in the reference light beamincident on the detector is 25% of the light energy from the source andis higher than the sample light energy. To prevent saturation of thedetector, the reference beam has to be suppressed. In the embodiments ofFIGS. 3 a and 4, in the sample arm, the light source energy is reducedby 75% by two passes through the 50/50 beam splitter and then by 10% bythe 10/90 beam splitter. In the reference arm, the light source energyis reduced by 50% by the first beam splitter, 50% by the diffractiongrating and 90% by the second beam splitter. In the embodiment of FIG.5, in the sample arm, the light source energy is reduced by 10% by the90/10 beam splitter and by 37% by two passes through the opticalcirculator. The loss caused by the 50/50 beam splitter does not reducethe total energy of the sample light beam because the total energy ofthe light incident on both detectors by the sample light beam is thesame as the energy of the sample light beam incident on the beamsplitter. In the reference arm, the light is reduced to 10% of the lightenergy from the source by the 10/90 beam splitter and then by 50% by thediffraction grating. In the embodiments of FIGS. 3 a, 4 and 5, theproportion of the initial light energy in the reference light beamincident on the detector is much lower than in the prior art and theproportion of the light energy in the sample light beam is higher.Saturation of the detector or detectors may be readily avoided bysuitable selection of the characteristics of the beam splitters. Aneutral density filter may be provided along the reference arm for moreprecise control over the energy of the reference light beam, ifnecessary. Since more of the light energy from the source may beallocated to the sample light beam, where it is most needed, less energyis wasted in the system.

FIG. 9 is an enlarged view of the reference light beam R emitted by thecollimator 116 being diffracted by the diffraction grating 118, showingthe maximum optical path difference 6 across the diffracted referencelight beam Rd for the embodiment of FIG. 3 a. The second sample lightbeam S received from the sample is shown being emitted by the collimator112. The second beam splitter 120 is also shown. The optical pathdifference δ varies gradually across the diffracted reference light beamRd such that the difference at one side of the beam cross-section “a” isabout zero and the difference at the opposite side of the beam “b” isthe maximum difference δ. The maximum optical path difference δ istypically chosen to enable measurement of the light scattered from thedesired depth. Since the optical paths of the reference and sample lightbeams have to be substantially equal, the optical path difference δcorresponds to the depth of the image in the second sample light beam S,corrected by the refractive index of the media in which the depth ismeasured. The maximum optical path difference δ is a function of thewidth Wd of the diffracted light beam and the angle of incidence α:δ=Wd×Sin α.  (1)

The depth Δ is a function of the maximum optical path difference δ.Since it takes a two-way sample beam path to determine the depth versusa one way reference beam path to define the maximum optical pathdifference, the depth is half of the maximum optical path difference δ.Since the depth Δ is measured in a material other than air, it is also afunction of the refraction coefficient of the sample material η:Δ=δ/2η.  (2)

The angle of incidence α of the reference light beam on the diffractiongrating is a function of the diffraction grating parameter p (distancebetween adjacent grooves) and the light wavelength λ. The diffractiongrating formula is:Sin α=λ/p.  (3)

Also, as shown in FIG. 9, the width Wref of the reference light beam Ris less than the width Wd of the diffracted reference light beam Rd.Preferably, the width Wd of the diffracted reference light beam is thesame as the width Ws of the second sample light beam S. The combinedlight beam (not shown) has the same width. The width Wref of thereference light beam is therefore preferably:Wref=Ws/Cos α.  (4)

The width of the detector array or arrays should the same or slightlygreater than the width of the combined light beam. Preferably, the firstcollimator 112, that collimates the second sample light beam S receivedfrom the sample, has the same dimensions as that of the detection array.

For example, if the sample is biological tissue (η=1.33) and the depthof measurement is Δ=3 mm, the maximum optical path difference fromformula (2) would be: δ=

7.98 mm. If the light source has a wavelength λ=820 nm and thediffraction grating parameter is p=1/830 mm, the angle α from formula(3) would be: Sin α=0.697 (α=44.2 deg.). Then, the width Wd of thediffracted reference beam Rd, which is preferably equal to the width Wsof the second sample light beam received from the sample Ws from (1)would be Wd=11.45 mm. The width of the combined light beam is also Wd.The photo detector array would then also have a width of at least 11.45mm.

In the embodiments above, the light source 102 is a low coherence,broadband light source, such as a super luminescent diode. The coherencelength of the light source may be from about 15 to about 30 microns, forexample. The wavelength may be between about 800 to about 1500nanometers, for use with biological tissue. The light source should emitlight at a power of at least about 10 milliwatts for depth measurementsof about 1 millimeter. The light source should emit light at a power ofat least about 50 milliwatts for depth measurements of 2-3 millimeters.Superluminescent diodes for use in the embodiments may be obtained fromSuper Lume Diodes, Ltd. Moscow, Russia, or Hamamatsu Photonics K.K.,Solid State Division, Hamamatsu City, Japan, (“Hamamatsu”) for example.

The detector is preferably a multi-element photo detector, such as aphoto diode array. An avalanche mode photo diode array may be used, forexample The photodiode array preferably has at least 256 diodes. Anarray of 512 photo diodes or more is more preferred. Photo diode arraysmay be obtained from Sensors Unlimited, Inc., Princeton, N.J. andHamamatsu, for example. A charge-coupled device (“CCD”) may also beused.

Appropriate optical fibers and fiber optic beam splitters of desiredcharacteristics are readily commercially available. They may be obtainedfrom Corning Incorporated, Corning, N.Y., for example. Open space beamsplitters of desired characteristics are also readily commerciallyavailable. They may be obtained from Edmunds Scientific, Tonawanda,N.Y., for example. The conjugating lenses and focussing lens may also beobtained from Edmunds Scientific, for example.

FIG. 10 is yet another embodiment of an interferometric system 300,wherein an acousto-optic modulator (“AOM”) 302 acts as both atransparent diffraction grating to introduce an optical path differenceto the reference light beam and as a modulator to introduce a frequencyshift. Otherwise, the system is the same as the embodiment of FIG. 3 a.One AOM may be used for shallow depths of a few hundred microns, forexample. Two modulators may be used for greater depths of 500 to 1,000microns, for example. One AOM may also be used along with a transparentdiffraction grating, as shown in U.S. Pat. No. 6,114,645, which isincorporated by reference herein. While one AOM may introduce afrequency of modulation higher than that desirable in an OCT system, twoor more AOM's in series, each driven at different frequencies, may beused to achieve the desired frequency. The AOM 302 may be driven by aprogrammable signal generator, as is known in the art.

Since the projected interference pattern generated by theinterferometric system of the invention is formed on the detector nearlyinstantaneously, pulsed imaging may also be implemented in any of theembodiments discussed above. Pulsed imaging allows for the use of higherpeak power and lower average power (lower duty cycle), enablingincreased penetration through attenuative structures while maintaininglow average light energy for safe operation. A laser diode may be usedin a pulsed mode as the light source in any of the embodiments. Thelaser diode may be smaller and less expensive than the superluminescentdiode discussed above for continuous operation, because a small laserdiode may produce a sufficient peak output at a wider bandwidth in apulse mode without being destroyed.

As discussed above, the output of the detector or detectors in theembodiments above may be analyzed in a conventional manner to produce animage. The output may also be analyzed by the same algorithms used toprocess ultrasound data. FIG. 11 is a schematic diagram of an imagingsystem 390 comprising an interferometer 391 including a photo detectorarray 392 with a plurality of parallel outputs 394 connected to anultrasound console 396 through a parallel to serial converter 398. Anultrasound device 399 is also shown, with an output 399 a. The output399 a of the ultrasound device 399 may also be connected to theultrasound console, in the same or a different input than theinterferometer 391. A doctor or technician may thereby use either theinterferometer 391 for optical imaging or the ultrasound device 399 forultrasound imaging, with the same ultrasound console. The parallel toserial converter 398 is discussed further below. First, ultrasoundimaging is briefly discussed.

Ultrasound medical imaging is a commonly used procedure to produceimages of body cavities such as blood vessels and surrounding tissue. Toimage a blood vessel and surrounding tissue through ultrasound, anIntravascular Ultrasound (“IVUS”) catheter is typically inserted intothe blood vessel in a known manner. An example of an IVUS catheter maybe found in U.S. Pat. No. 5,715,825, entitled Acoustic Imaging Catheterand the Like, incorporated by reference herein.

In ultrasound imaging, an ultrasound transducer is supported at thedistal end of an IVUS catheter, for example. The transducer emitsultrasound waves in the blood vessel or other such cavity when excitedby a pulse. A portion of the emitted ultrasound waves is reflected backto the ultrasound transducer by tissue boundaries. The reflectedultrasound waves induce an echo signal at the ultrasound transducer. Theecho signal is transmitted from the ultrasound transducer to anultrasound console, which typically includes an ultrasound imageprocessor, such as a computer, a microprocessor or a microcontroller,and a display. The display may comprise a monitor and/or a printer. Theultrasound console uses the received echo signal to image the cavity. Anultrasound system including an ultrasound image processor and display isavailable from Boston Scientific Corporation, Natick, Mass.

The echo signal is a serial amplitude modulated signal in which theamplitude of the signal varies with time. A typical echo signal has atime length of 8 μs, which corresponds to an image depth ofapproximately 6 millimeters from the ultrasound transducer. The echosignal carries both image brightness information and image depthinformation, where depth may be taken with respect to the ultrasoundtransducer. The image brightness information is provided by theamplitude of the echo signal. The image depth information is provided bythe time position within the echo signal. An earlier time position inthe echo signal corresponds to a lower image depth than a later timeposition in the echo signal.

In applicants' improved interferometric systems discussed above, as wellas in other OCT systems using an array of photo detectors, the arraycaptures image brightness information at multiple image depths in oneinstance. Since the detected spatial information may be read and stored,the parallel channel outputs of the photo detector array of theinterferometric system may be processed into a serial analog or serialdigital signal by a parallel to serial converter. The resulting serialsignal carries image brightness information and image depth informationin a similar manner as a typical echo signal. The time length and/orfrequency of the serial signal may be adjusted to better match the timelength and/or frequency of a typical echo signal that the ultrasoundconsole is configured to receive by synchronizing the signal to thesweep speed of the ultrasound device (speed of rotation of thetransducer) and to the propagation velocity of sound. This enables anultrasound console to process the serial analog signal into an image, inthe same way ultrasound data is processed. The same ultrasound consolemay thereby be used to process both ultrasound based images derived fromdata received from an ultrasound catheter and optical interferometricbased images derived from data received from an interferometriccatheter, thereby reducing the cost of having both ultrasound imagingand optical imaging capabilities.

FIG. 12 shows an AOM based interferometric system 400, as in theembodiment of FIG. 10, wherein the first, fiber optic beam splitter 202is a 90/10 beam splitter and the second, open space beam splitter 222 isa 50/50 beam splitter. Two photo detector arrays 224, 226 are provided,as in the embodiment of FIG. 5. The outputs of corresponding detectorsin each of the parallel outputs from the photo detector arrays 224, 226of the interferometric system 400 are combined by analog circuitry 227,as discussed. The parallel outputs of the analog circuitry 227 are inputto a parallel to serial converter 398 that converts the parallel outputsinto a serial amplitude modulated signal that can be processed by anultrasound console. If only one detector is provided, as in theembodiment of FIG. 3 a, for example, the analog circuitry 227 is notneeded and the parallel outputs of the photo detector array 122 could beprovided directly to the parallel to serial converter 398.

A computer 404 may optionally be provided to process the serial signaloutput by the parallel to serial converter. The serial signal is thenprovided to the IVUS console 396 through an input 405 for processinginto an image for display. The IVUS console comprises a signalprocessor, such as a computer, a microprocessor or a microcontroller,and a display to display generated images, as is known in the art. Theinterferometer may be selectively connected to the input 405 whenoptical imaging is desired. An ultrasound catheter 399 is also shown inFIG. 12, with an output 399 a. The output 399 a may be connected to theinput 405 or a separate input 417 of the IVUS console 396 whenultrasound imaging is desired.

The other embodiments of the interferometric systems, as well as otherinterferometric systems, could be used with the IVUS console, as well.

The optical fiber 208 of the sample arm is shown coupled to an opticalfiber 407 within a catheter 408 through a rotary connector 410. A mirroror prism 412 is shown for reflecting the sample light beam out of thecatheter to tissue in a body cavity, as described above. The rotaryconnector 410 is driven by a motor, as is known in the art.

The parallel to serial converter may be one of the electronic interfacesdescribed in U.S. patent application Ser. No. 09/909,357 (“the '357application”), entitled “Electronics Interface for an UltrasoundConsole”, filed on Jul. 18, 2001, assigned to the assignee of theinvention and incorporated by reference herein.

The ultrasound console 396 may be configured to receive either an analogor a digital input. In one example of an electronics interface disclosedin the '357 application for receiving a analog input, the electronicsinterface comprises a plurality of channel processors, each coupled toone of the parallel channel outputs of the photo array. Each channelprocessor comprises an analog processor, an A/D converter, aFirst-In-First-Out (“FIFO”) memory buffer and a data bus coupled to theFIFO memory buffer of each one of the channel processors. A single FIFOmemory buffer is coupled to the data bus and a D/A converter is coupledto the output of the single FIFO memory buffer. The output of the D/Aconverter is coupled to the input of the ultrasound console. Acontroller coupled to an ultrasound motor encoder synchronizes theoperation of the electronics interface with the ultrasound console,based on the rotation of the motor rotating the rotary connector 410coupled to the optical fiber 407 within the catheter 408. The operationof the interface is described in more detail in the '357 application.

Where the ultrasound console is adapted to receive a digital input, theserial digital data sequence from the single FIFO memory is provided tothe input of the console through control logic that controls thetransfer of the digital data sequence, as is also described in the '357application.

The photo detector array may be a multiplexed photo detector array.Electrical interfaces for single and double channel multiplexed photodetector arrays are also described in the '357 application.

Other electronic interfaces for converting the parallel output of thearray of photo detectors into a serial analog or digital data stream,may also be used.

FIG. 13 shows a housing 420 containing an interferometer in accordancewith the embodiment of FIG. 3 a of the present invention for use with aninterferometric catheter 408 and an IVUS console 396. Components commonto the other embodiments are commonly numbered. The light source 102,the fiber optic beam splitter 104, the optical fibers 106, 108, 110 and114, the diffraction grating 118, the collimators 112, 116, the openspace beam splitter 120, the conjugating lens 124 and the multi-elementdetector 122 are shown. The rotary coupler 410 of FIG. 11 is also shown.A motor 422 is provided in the housing 420 to rotate the rotary coupler410 and the optical fiber 407 within the catheter 408. A motorcontroller 424 controls the operation of the motor 422. A power supply426 is shown, as well. Data acquisition and processing boards 428 areprovided, electrically connected to a cable 430 for connection to theIVUS console 396 of FIG. 11. The parallel to serial converter 398 inFIG. 12 may be included on the processing boards. A port 428 of thehousing and a catheter adapter 430 for connection to the port are shownas well.

As mentioned above, any of the embodiments of the interferometricsystems described herein, as well as other fiber optic and non fiberoptic OCT systems using a multi-element photo detector, may be used inthe imaging system 390. For example, the systems of U.S. Pat. No.5,943,133 and “Nonmechanical grating-generated scanning coherencemicroscopy”, Optics Letters, Vol. 23, No. 23, Dec. 1, 1998, discussedabove and incorporated by reference herein, in their entirety, may alsobe used.

Alternatively, an oscillating mirror or other such reflector may be usedto scan a sample depth. For example, interferometers such as thosedescribed in U.S. Pat. No. 6,134,003, U.S. Pat. No. 6,111,645, U.S. Pat.No. 5,459,570, U.S. Pat. No. 5,321,501 and International Publication No.WO 98/38907 published on Sep. 11, 1998, for example, which are alsoincorporated by reference herein, may also be used in the imaging systemwith an interface disclosed in the '903 application or other suchinterfaces.

Another interferometric system which may be used in the imaging system390 is described in U.S. patent application Ser. No. 09/906,903,entitled “Electronically Scanned Optical Coherence Tomography withFrequency Modulated Signals”, filed on Jul. 16, 2001, assigned to theassignee of the present invention and incorporated by reference herein.There, an interferometer uses a single element detector and image depthinformation is carried on multiple modulation frequencies, eachcorresponding to a different depth. The image depth information in thesignal output by the detector may be resolved by tuning to the desiredfrequency. Interfaces for coupling such interferometers to an IVUSconsole are also disclosed.

As was also mentioned above, the sample arm may be incorporated in anendoscope for insertion into the gastrointestinal tract, for example.The sample arm may also have a probe at its end for examining externalbiological tissue, such as the eye, or other types of samples, such assemiconductors.

While the preferred embodiments described above are implemented withfiber optics for use in examining internal biological tissue, such asbiological tissue along internal body cavities and organs, theembodiments of the invention may be readily implemented with bulk opticsor other optical components. For example, in examining semiconductorsand external biological tissue, a non fiber optic interferometer may beused in accordance with the invention. In a non fiber opticimplementation, one collimator is preferably provided between the lightsource and the first beam splitter.

While use of a multi-element photo detector array is preferred, a singleelement photo detector may also be used, in which case the width of thecombined light beam could be moved across the detector or the detectorcould be moved across the width of the combined light beam.

Use of a focusing lens, first and second collimators and one or twoconjugating lenses are also preferred, but not required.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that modifications maybe made to those embodiments without going beyond the spirit and scopeof the invention, as defined by the following claims and theirequivalents.

1. An interferometer comprising: a low coherence light source; a firstoptical fiber coupling the light source to the first beam splitter,wherein the first beam splitter splits light received from the lightsource into a sample beam and a reference beam; an optical circulatorhaving a first port, a second port, and a third port, wherein lightinput to the first port exits the optical circulator from the secondport and light entering the second port exits the optical circulatorfrom the third port; a second optical fiber optically coupling the firstbeam splitter to the first port of the optical circulator; a thirdoptical fiber to convey the sample light beam to a sample and to conveya second sample light beam received from the sample to the first beamsplitter; a second beam splitter; a fourth optical fiber opticallycoupling the third port of the optical connector to the second beamsplitter, wherein the third optical fiber conveys the second samplelight beam, at least in part, from the third port to the second beamsplitter; a diffraction grating; a fifth optical fiber opticallycoupling the first beam splitter to the diffraction grating to conveythe reference light beam, at least in part, to the diffraction grating;the second beam splitter being positioned to receive the diffractedreference light beam from the diffraction grating, wherein the referencelight beam and the second sample light beam combine in the beam splitterto form first and second combined light beams; and first and seconddetectors positioned to receive the first and second combined lightbeams, wherein each detector has an output.
 2. The interferometer ofclaim 1, further comprising analog circuitry, having parallel outputs,coupled to the outputs of the first and second detectors.
 3. Theinterferometer of claim 2, further comprising a parallel to serialconverter coupled to the parallel outputs of the analog circuitry,wherein the parallel to serial converter is configured to be coupledwith an ultrasound console.
 4. The interferometer of claim 1, whereinthe third optical fiber is optically coupled to the second port of theoptical circulator and a rotary connector having an output coupled witha mirror.
 5. The interferometer of claim 4, wherein the third opticalfiber is located within a catheter.
 6. The interferometer of claim 1,wherein the third optical fiber optically coupled to the second port ofthe optical circulator and a rotary connector having an output coupledwith a prism.
 7. The interferometer of claim 1, further comprising aprocessor communicatively coupled with the outputs of the first andsecond detectors.
 8. The interferometer of claim 1, wherein the lightreceived from the light source has an energy and the first beam splittersplits the light into a sample light beam having more than half of theenergy of the light and a reference light beam having less than half ofthe energy of the light.
 9. The interferometer of claim 8, furthercomprising: a focusing lens to focus the sample light beam onto thesample and to focus the reflected sample light beam; a first collimatoroptically coupled between the fourth optical fiber and the second beamsplitter such that the fourth optical fiber conveys the reflected samplelight beam to the first collimator to collimate the reflected samplelight beam and the collimated sample light beam is directed to thesecond beam splitter; a second collimator optically coupled between thefifth optical fiber and the diffraction grating such that the fifthoptical fiber conveys the reference light beam to the second collimatorto collimate the reference light beam and the collimated reference lightbeam is directed onto the diffraction grating; and first and secondconjugating lenses between the second beam splitter and the first andsecond detectors.
 10. The interferometer of claim 1, further comprisingfirst and second conjugating lens between the first detector and thesecond beam splitter and the second detector and the second beamsplitter, respectively.
 11. The interferometer of claim 1, wherein thefirst and second detectors are each a multi-element photo detector. 12.The interferometer of claim 1, further comprising first and secondpolarization filters positioned to filter the first and second combinedlight beams, respectively, with respect to first and second respectivepolarizations.
 13. The interferometer of claim 1, further comprising: asecond light source optically coupled to the first optical fiber, thesecond light source emitting light at a wavelength different than thewavelength of the first light source; wherein the first detector detectslight at a wavelength corresponding to the wavelength of the lightemitted by the first light source and the second detector detects lightat a wavelength corresponding to the wavelength of the light emitted bythe second light source.
 14. The interferometer of claim 13, wherein oneof the light sources emits light in a wavelength band that inducesfluorescence in the sample.
 15. The interferometer of claim 1, whereinthe second beam splitter directs more than half of the energy in thereflected sample light beam into the combined beam and less than half ofthe energy in the reference light beam into the combined beam.
 16. Theinterferometer of claim 1, further comprising a phase modulator tomodulate either one of the reference light beam and the reflected samplelight beam.
 17. The interferometer of claim 1, wherein the diffractinggrating is a reflective diffraction grating, a transparent diffractiongrating, or an acousto-optic modulator.
 18. The interferometer of claim1, further comprising a catheter, wherein at least a portion of thethird optical fiber is within the catheter.
 19. The interferometer ofclaim 1, further comprising: a signal processor electrically connectedto the first and second detectors to receive an output from the firstand second detectors and to process the signals.
 20. The interferometerof claim 1, wherein the light source is pulsed.
 21. The interferometerof claim 1, wherein the first beam splitter splits the light receivedfrom the light source into a sample light beam having substantially morethan half of the energy of the light and a reference light beam havingsubstantially less than half of the energy of the light.
 22. Theinterferometer of claim 21, wherein the first beam splitter directs atleast about 90% of the light energy received from the light source intothe sample light beam and about 10% or less of the light energy receivedfrom the light source into the reference light beam.
 23. Theinterferometer of claim 1, wherein the first beam splitter splits thelight received from the light source into a sample light beam havingsubstantially more than half of the energy of the light and a referencelight beam having substantially less than half of the energy of thelight.
 24. The interferometer of claim 1, wherein the second beamsplitter directs at least about 90% of the light energy received fromthe light source into the sample light beam and about 10% or less of thelight energy received from the light source into the reference lightbeam.